Install hooks by copying into .git/hooks instead of core.hooksPath

Linking core.hooksPath to the tracked, sandbox-writable .githooks/ let a
sandboxed command rewrite a hook that git commit/push then run unsandboxed.
Copies in .git/hooks can't be changed from the sandbox. This also keeps
existing .git/hooks scripts working and lets npm install succeed outside a
git checkout. pre-merge-commit now names itself in errors; hook tests clean
up, ignore GIT_* env and cover the installer; docs state what each layer
does and doesn't stop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.githooks/pre-commit

@@ -1,7 +1,10 @@
 #!/bin/sh
 # Refuse commits (and, via pre-merge-commit, merge commits) while main is checked out.
+# Cherry-pick, revert and fast-forward merges don't run these hooks; pre-push still stops
+# their result from reaching the remote.
+name=${1:-pre-commit}
 if [ "$(git symbolic-ref --short -q HEAD)" = "main" ]; then
-  echo "$(basename "$0"): committing on main is not allowed; create a branch first." >&2
+  echo "$name: committing on main is not allowed; create a branch first." >&2
   exit 1
 fi
 exit 0

.githooks/pre-merge-commit

@@ -1,2 +1,2 @@
 #!/bin/sh
-exec "$(dirname "$0")/pre-commit"
+exec sh "$(dirname "$0")/pre-commit" pre-merge-commit

CLAUDE.md

@@ -3,7 +3,8 @@
 A tiny zero-dependency Node.js todo CLI, used as a sandbox for trying Claude Code features.
 
 ## Commands
-- `npm test` — run the test suite (built-in `node:test`, no install needed)
+- `npm install` — once per clone (no dependencies): installs the git hooks that block commits and pushes to `main`. If it fails in the sandbox, ask the human to run it.
+- `npm test` — run the test suite (built-in `node:test`)
 - `npm start -- add "something"` / `npm start -- ls` — run the CLI (data in `todos.json`, override with `TODO_FILE`)
 
 ## Conventions

README.md

@@ -3,7 +3,7 @@
 A tiny todo CLI that exists to try out [Claude Code](https://docs.claude.com/en/docs/claude-code) features.
 
 ```sh
-npm install   # once per clone: turns on the git hooks in .githooks/
+npm install   # once per clone, and after changing .githooks/: installs the git hooks
 npm test
 npm start -- add "try Claude Code"
 npm start -- done 1
@@ -22,9 +22,11 @@ npm start -- ls
 ### Protecting `main`
 Changes reach `main` only through a PR that a human merges. Three layers, strongest first:
 1. **Gitea branch protection** on `main` (set in the repo settings on code.westerkam.org): the only layer that can't be bypassed from a clone.
-2. **Git hooks** in `.githooks/` (enabled by `npm install`): `pre-push` rejects any push that would update `main`, however it's spelled; `pre-commit` / `pre-merge-commit` refuse commits while `main` is checked out. `--no-verify` skips them.
+2. **Git hooks** from `.githooks/`, copied into `.git/hooks` by `npm install` (`scripts/install-hooks.sh`). `pre-push` rejects any push that would update `main`, however it's spelled. `pre-commit` / `pre-merge-commit` refuse ordinary and merge commits while `main` is checked out; cherry-pick, revert and fast-forwards skip them, but pre-push still stops the result leaving the machine. The hooks are copied, not linked with `core.hooksPath`, because Claude Code's sandbox can't write inside `.git/`: a sandboxed command can't rewrite the installed hooks, which matters because `git commit` / `git push` run outside the sandbox. Anyone can still skip them with `--no-verify`.
 3. **Claude Code deny rules** in `.claude/settings.json` for `git push … main`: a quick early stop, but they match command text, so they're easy to get around.
 
+Layers 2 and 3 live on your machine and can be bypassed by whoever runs the commands; only layer 1 can't.
+
 ## Things to try
 
 - *"There's a bug in how ids are assigned after removing a todo — find it and fix it with a test."*

package.json

@@ -7,7 +7,7 @@
   "scripts": {
     "start": "node src/cli.js",
     "test": "node --test",
-    "prepare": "git config core.hooksPath .githooks"
+    "prepare": "sh scripts/install-hooks.sh"
   },
   "engines": { "node": ">=20" },
   "license": "MIT"

scripts/install-hooks.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# Copy the hooks in .githooks/ into git's own hooks directory. They are copied rather than
+# enabled with core.hooksPath on purpose: Claude Code's sandbox can't write inside .git/, so
+# a sandboxed command can't rewrite the installed hooks, even though git commit and git push
+# (and so their hooks) run outside the sandbox. Re-run after changing anything in .githooks/.
+set -e
+cd "$(dirname "$0")/.."
+if ! hooks=$(git rev-parse --git-path hooks 2>/dev/null); then
+  echo "install-hooks: not a git checkout; skipping." >&2
+  exit 0
+fi
+mkdir -p "$hooks"
+for hook in .githooks/*; do
+  cp "$hook" "$hooks/"
+  chmod +x "$hooks/$(basename "$hook")"
+done
+echo "install-hooks: installed $(ls .githooks | tr '\n' ' ')into $hooks"

test/hooks.test.js

@@ -1,28 +1,37 @@
-import { test } from "node:test";
+import { test, after } from "node:test";
 import assert from "node:assert/strict";
 import { spawnSync } from "node:child_process";
-import { mkdtempSync } from "node:fs";
+import { mkdtempSync, rmSync, cpSync, statSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { fileURLToPath } from "node:url";
 
-const HOOKS = fileURLToPath(new URL("../.githooks/", import.meta.url));
+const ROOT = fileURLToPath(new URL("../", import.meta.url));
+const HOOKS = join(ROOT, ".githooks");
 const SHA = "1".repeat(40);
 const ZERO = "0".repeat(40);
 
-function prePush(...lines) {
-  return spawnSync("sh", [join(HOOKS, "pre-push"), "origin", "git@example:repo.git"], {
-    input: lines.map((l) => l + "\n").join(""),
-    encoding: "utf8",
-  });
-}
+// Run git and the hooks without inheriting GIT_DIR & co, e.g. when tests run from inside a hook.
+const env = Object.fromEntries(Object.entries(process.env).filter(([k]) => !k.startsWith("GIT_")));
+const run = (cmd, args, opts = {}) => spawnSync(cmd, args, { env, encoding: "utf8", ...opts });
+
+const dirs = [];
+after(() => dirs.forEach((d) => rmSync(d, { recursive: true, force: true })));
 
 function repoOn(branch) {
   const dir = mkdtempSync(join(tmpdir(), "todo-hooks-"));
-  spawnSync("git", ["init", "-q", "-b", branch, dir]);
+  dirs.push(dir);
+  const init = run("git", ["init", "-q", "-b", branch, dir]);
+  assert.equal(init.status, 0, `git init failed: ${init.stderr}`);
   return dir;
 }
 
+function prePush(...lines) {
+  return run("sh", [join(HOOKS, "pre-push"), "origin", "git@example:repo.git"], {
+    input: lines.map((l) => l + "\n").join(""),
+  });
+}
+
 for (const [label, line] of [
   ["an update", `refs/heads/fix/x ${SHA} refs/heads/main ${SHA}`],
   ["a new ref", `refs/heads/fix/x ${SHA} refs/heads/main ${ZERO}`],
@@ -46,10 +55,30 @@ test("pre-push allows other branches, including ones named like main", () => {
 });
 
 for (const hook of ["pre-commit", "pre-merge-commit"]) {
-  test(`${hook} rejects main and allows other branches`, () => {
-    const onMain = spawnSync("sh", [join(HOOKS, hook)], { cwd: repoOn("main"), encoding: "utf8" });
+  test(`${hook} rejects main, names itself, and allows other branches`, () => {
+    const onMain = run("sh", [join(HOOKS, hook)], { cwd: repoOn("main") });
     assert.equal(onMain.status, 1);
-    assert.match(onMain.stderr, /committing on main is not allowed/);
-    assert.equal(spawnSync("sh", [join(HOOKS, hook)], { cwd: repoOn("fix/x") }).status, 0);
+    assert.match(onMain.stderr, new RegExp(`^${hook}: committing on main is not allowed`));
+    assert.equal(run("sh", [join(HOOKS, hook)], { cwd: repoOn("fix/x") }).status, 0);
   });
 }
+
+test("install-hooks copies every hook into .git/hooks as an executable", () => {
+  const repo = repoOn("fix/x");
+  cpSync(join(ROOT, "scripts"), join(repo, "scripts"), { recursive: true });
+  cpSync(HOOKS, join(repo, ".githooks"), { recursive: true });
+  const res = run("sh", [join(repo, "scripts", "install-hooks.sh")]);
+  assert.equal(res.status, 0, res.stderr);
+  for (const hook of ["pre-push", "pre-commit", "pre-merge-commit"]) {
+    assert.ok(statSync(join(repo, ".git", "hooks", hook)).mode & 0o100, `${hook} is not executable`);
+  }
+});
+
+test("install-hooks skips cleanly outside a git checkout", () => {
+  const dir = mkdtempSync(join(tmpdir(), "todo-hooks-"));
+  dirs.push(dir);
+  cpSync(join(ROOT, "scripts"), join(dir, "scripts"), { recursive: true });
+  const res = run("sh", [join(dir, "scripts", "install-hooks.sh")], { env: { ...env, GIT_CEILING_DIRECTORIES: tmpdir() } });
+  assert.equal(res.status, 0);
+  assert.match(res.stderr, /not a git checkout; skipping/);
+});